Active pixel sensor (APS) devices are described in U.S. Pat. No. 5,471,515. These devices include an array of pixel units that convert light energy into electrical signals. Each pixel includes a photodetector and one or more active transistors.
Unlike charge coupled devices (CCD), APS imaging devices are compatible with complementary metal oxide semiconductor (CMOS) technology. This feature enables integration of imaging electronics and digital image signal processing electronics on the same chip in APS devices. Such integration can provide advantages in design and performance.
Conventional CMOS imaging devices can be prone to producing artifacts in the resulting image. Artifacts are objects in the image that appear black when in fact they should be the brightest regions of the image.
Artifacts can occur in oversaturated pixels in the APS. Oversaturated pixels are pixels that are exposed to more light energy than the pixel's photodetector can absorb during exposure time.
This is mainly a problem in very high contrast images. The extreme brightness of the sun will produce artifacts in virtually all CMOS imagers, even at very short integration times. This is a disconcerting effect in outdoor imaging situations, in which the sun is often accidentally included in the field of view.
FIG. 1 illustrates such a situation in a digital image 10 produced using a conventional APS CMOS imager. The presence of the sun 12 in the image produces an artifact 14 in which the center of the sun appears black.